The ATP-binding cassette (ABC) gene family encodes a diverse group of transporter proteins that pump a wide variety of compounds across the membranes of cells and tissues. Human ABC genes are involved in a number of diseases, including cystic fibrosis, adrenoleuko-dystrophy, and familial persistent hyperinsulinemic hypoglycemia. Resistance of tumors to multiple drugs (multidrug resistance, MDR) is a major limitation of cancer chemotherapy. MDR is associated in certain tumors with the overexpression of the P-glycoprotein/(PGP)MDR and multidrug resistance- related protein (MRP) genes. PGP and MRP are ABC family transporters. We have previously identified over 21 new human ABC genes. The genetic location of each of these genes as well as their expression pattern has been determined. One of these genes, ABCR, is expressed exclusively in the retina. The gene was mapped to chromosome 1p13- 21, the location of the Stargardt macular dystrophy (STGD1) gene, a recessive disorder causing vision loss in children. We identified a total of 19 different mutations in the ABCR gene in STGD1 patients, including several frame- shift and non-sense alleles. STGD1 is phenotypically similar to age- related macular degeneration (AMD), a common form of retinal degeneration that occurs in people over age 65. We examined a cohort of 167 AMD patients and found that 16% of them had mutations in the ABCR gene. Understanding the role of mutations in this gene in STGD1 and AMD, and identifying the normal function of ABCR should shed light on the development of macular degeneration. Further characterization of other ABC genes may provide information on other human diseases and neoplasms.